Neurogenesis is defined as generation and maturation of new neurons. Postnatal neurogenesis, a process considered important for neuroplasticity and memory, is regulated at multiple molecular levels. Deciphering these regulatory mechanisms represents a step towards understanding the development and plasticity of postnatal mammalian brains, and realizing the therapeutic potential of neural stem/progenitor cells (NSCs). Epigenetic mechanisms, including DNA methylation and histone modification, are known to play significant roles in the modulation of stem cell proliferation and differentiation. Methyl-CpG binding proteins, including MBD1 and MeCP2, are central players in epigenetic regulation, and can translate DNA methylation into gene expression changes. MBD1 deficiency has been reported in sporadic human cancers, consistent with its role in cellular growth control. Despite its ubiquitous expression pattern, we found that MBD1 deficiency in mice results largely in brain-associated phenotypes during the postnatal period, including impaired adult neurogenesis and related behavioral deficits such as defective hippocampus-dependent learning and susceptibility to anxiety and depression. Recently, MBD1 mutations were found in a subset of autistic patients and were correlated with more severe phenotypes. However, the precise role of MBD1 in postnatal neuronal development and molecular pathway mediating its effect is not fully clear. During the past three-year funding period, we have discovered that MBD1 regulates the expression of a number of miRNAs and some of these miRNAs exhibit an important regulatory role in neurogenesis. For example, miR-184 promotes proliferation and represses differentiation of adult NSCs by repressing the expression of Numblike (Nbl), a regulator of Notch signaling. The complete picture of this regulatory network is still lacking. In addition to its role in NSC proliferation and neuronal differentiation, we discovered that MBD1 also had important roles in maturation of new neurons. Indeed, some of MBD1-regulated miRNAs have been implicated in neuronal maturation. Taken together, these breakthrough discoveries serve as the basis of this proposal which is aimed towards a better understanding of the epigenetic mechanisms controlling multiple stages of postnatal neurogenesis. We will test the hypothesis that MBD1 regulation of miRNAs and their subsequent downstream targets is critical for postnatal neurogenesis. Therefore we propose to determine how MBD1-regulated miRNAs govern the proliferation and differentiation of aNSCs (Aim 1), to determine whether and how MBD1-regulated miRNAs modulate the maturation of new neurons (Aim 2), and to explore the mechanism underlying MBD1 regulation of small RNAs (Aim 3). The results will provide novel insights into the epigenetic mechanisms governing postnatal neurogenesis.